|A/C ORIFICE, ACCUMULATOR, & RECEIVER DRIER
by Jim Miller
Troubleshooting and repairing automotive A/C systems typically involves inspection of the following key A/C components.
A/C ORIFICE TUBE
An orifice tube is used to control the flow of refrigerant into the evaporator. It works in conjunction with compressor controls to maintain low side pressure at a sufficient level, to prevent evaporator icing or flooding. The orifice tube is placed in the high side liquid refrigerant line, at or near the evaporator. Some system designs place the orifice tube in the condensor outlet to dampen the hissing noise produced when the system is shut down. The orifice tube contains a small brass tube machined to precise diameter, that is used to meter refrigerant flow. The tube is set inside a plastic carrier
which also houses a mesh filter screen that is used to keep debris from entering the orifice. Seals are placed on the outside of the orifice tube assembly to block refrigerant from flowing around the tube and to help secure it in the liquid line.
Automotive air conditioning systems that use an orifice tube also must use compressor controls to maintain low side pressure and temperature. Most use pressure cycling switches that are mounted on the accumulator to shut the compressor off when low side pressure drops below a specified level. Some older designs use a thermostatic cycling switch to control system pressure. These switches monitor low side temperature and cycle the compressor off when it drops below a specific level. The frequency of compressor cycling is dependent upon ambient air temperature and air flow across the condensor. Running the air conditioner at highway speeds or during cool days can result in the compressor cycling on and off at a higher rate. Compressor cycling may be felt by the operator of the vehicle, especially in vehicles equipped with small displacement engines. To address this, variable displacement compressors were designed to eliminate compressor cycling. Variable displacement compressors control system pressure by changing the amount of refrigerant the compressor can deliver in response to system demand. Refer to an auto repair manual for the specs on your vehicle’s A/C orifice and it’s removal and replacement instructions.
The accumulator is placed at the evaporator outlet and is used to separate liquid refrigerant from vaporized refrigerant, as well as remove any moisture contained in the refrigerant. Refrigerant enters the accumulator as it leaves the evaporator. Any liquid refrigerant is trapped in the accumulator, while vaporized refrigerant and refrigerant oil is allowed to pass on to the compressor. A desiccant bag is placed in the accumulator to remove any moisture that may have contaminated the air conditioning system.
A/C RECEIVER DRIER
The receiver drier is used in air conditioning systems that utilize an expansion valve for refrigerant flow control. The receiver drier stores half of the systems refrigerant charge and contains a desiccant bag to remove any moisture that may have contaminated the system. The receiver drier is placed in the high pressure liquid line, between the condensor and the expansion valve. It may contain a sight glass that is used as an indicator for the amount of refrigerant contained in the system. Pressure switches and a pressure relief valve may also be placed on the receiver drier.
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